1. Field of the Invention
This invention relates generally to a spectrum analyzer and, more particularly, is directed to a spectrum analyzer which is particularly suited for use in a graphic equalizer.
2. Description of the Prior Art
When reproducing an audio signal in a special sound field, such as, in an automobile, it is known to compensate for the sound field by employing a graphic equalizer, for example, as shown in the Japanese Published Patent Application No. 86913/1984. In such a graphic equalizer, the reproduced audio signal is divided into a number of frequency bands, and the signal levels of the divided bands are adjusted relative to each other so as to compensate for the sound field. A graphic equalizer of the described type usually has a spectrum analyzer associated therewith for displaying the signal levels of the several frequency bands in the form of respective bar graphs. In that case, as the signal levels of the several frequency bands are adjusted, the relationship of the adjusted signal levels can be ascertained by observing the bar graphs of the spectrum analyzer.
A spectrum analyzer providing bar graph displays of the signal levels of the several frequency bands in a conventional graphic equalizer is shown in FIG. 1 to include an input terminal 51 from which an input signal, for example, an audio signal reproduced from a tape cassette, is supplied to a multiband band-pass filter circuit 52. The multiband band-pass filter circuit 52 comprises a plurality of band-pass filters 52A-52J having respective pass bands with different center frequencies. More specifically, in the illustrated example of the prior art, the ten band-pass filters 52A-52J making up the multiband band-pass filter circuit 52 have center frequencies F.sub.11- F.sub.20, respectively. Thus, the audio signal from the input terminal 51 is divided by the multiband band-pass filter circuit 52 into ten components or frequency bands which are respectively centered about the frequencies F.sub.11 -F.sub.20.
Outputs of the band-pass filters 52A-52J are supplied to detecting circuits 53A-53J, respectively, which detect the signal levels of the ten frequency components or bands having the
frequencies F.sub.11 -F.sub.20, respectively. The detected signal level outputs from the detecting circuits 53A-53J are supplied to A/D converters 54A-54J, respectively, which provide digital data representing the signal levels of the frequency bands having the center frequencies F.sub.11 -F.sub.20, respectively. The digital data from the A/D converters 54A-54J are supplied to decoders 55A-55J, respectively, and each of the decoders 55A-55J converts the digital data supplied thereto and representing the signal level of a respective divided frequency band of the input signal into a decoded signal corresponding to one of a predetermined number, for example, thirteen, of steps or increments corresponding to respective dots or elements of an associated bar graph. The decoded outputs from the decoders 55A-55J are supplied through a driver 56 to a display device 57.
Such display device 57 may conventionally be in the form of a liquid crystal display (LCD) having a column of dots for each of the frequency bands into which the input signal is divided by the band-pass filter circuit 52, with each such column having a number of dots therein corresponding to the number of increments or steps into which each of the decoders 55A-55J converts the respective digital data from the A/D converter 54A-54J. Thus, in the illustrated spectrum analyzer according to the prior art, the LCD display 57 includes ten vertically extending columns l.sub.21 -l.sub.30 arranged in a horizontal series and each comprised of thirteen dots which are selectively excited, starting from the bottom of the respective column, for defining a bar graph representing the signal level of the respective frequency band divided from the input signal.
It will be appreciated that, with the spectrum analyzer according to the prior art as shown in FIG. 1, the adjusted levels of the several frequency components divided by the multiband band-pass filter circuit 52 so as to have center frequencies F.sub.11 -F.sub.20 are detected by the detecting circuits 53A-53J and then respectively displayed by the columns l.sub.21 -l.sub.30 of LCD dots or elements, as represented by the shaded areas on FIG. 1. Thus, in the illustrated example of the prior art, the input signal is divided into ten frequency components having the center frequencies F.sub.11 -F.sub.20, respectively, and the signal levels of the ten frequency bands are displayed by the ten bar graphs defined by the columns l.sub.21 -l.sub.30, respectively, of the display device 57. When fine adjustment of the frequency characteristic of an audio or other signal applied as the input to the terminal 51 is to be effected, it is desirable to increase the number of frequency bands which have their signal levels indicated by the display device 57. In other words, when only a relatively small number of frequency bands have their signal levels indicated by the display device 57, the displays of the frequency band levels are and it is difficult to visualize an image of the resulting frequency characteristic. If the number of frequency bands having their signal levels indicated by the display device 57 is increased, a curve representing the frequency characteristic of the reproduced audio signal can be more easily visualized from the increased number of bar graphs defined by the display 57.
However, in the spectrum analyzer of the prior art, the number of frequency bands or components which have their signal levels indicated by the display device 57 corresponds to the number of frequency bands divided by the multiband band-pass filter circuit 52. Therefore, in order to increase the number of frequency bands which are to have their signal levels indicated by the display device 57, it is necessary to correspondingly increase the number of band-pass filters included in the filter circuit 52. For example, in order to permit the signal levels of nineteen frequency bands or components to be displayed, it would be necessary to provide nineteen band-pass filters in the filter circuit 52. Furthermore, it would be necessary to similarly increase the numbers of detecting circuits, A/D converters and decoders provided in succession after the nineteen band-pass filters, respectively. Therefore, in the spectrum analyzer according to the prior art, an increase in the number of frequency bands which are to have their signal levels indicated by the display device 57 is achieved only at the expense of substantially increasing the complexity and cost of the associated circuit.